Descriptions

Marine bacterioplankton play an important role in global elemental cycles because they return carbon dioxide and nutrients to the biosphere as they reduce organic matter. Furthermore, marine bacterioplankton are not uniformly active, and subpopulations of the in situ community may be more or less active at any given time. Defining whether or not a cell is 'active' is not without difficulty, and the result varies depending on the assay used, since different assays examine different physiological processes within a cell. Linking the level of activity of a cell with its phylogenetic identity is an additional important step in examination of the role of marine prokaryotes in global elemental cycles. In this project, flow cytometry was used in two ways to examine relative cell-specific metabolic activity in bacterioplankton cells: as relative cell-specific nucleic acid content via staining with SYBR Green I, and as
ability to reduce sufficient 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) to be identified as having an active electron transport system. Based on flow cytometric sorting of cells labeled with ³H-leucine, the high nucleic acid (HNA) cells had higher cell-specific leucine incorporation rates than the low nucleic acid (LNA) cells. The HNA cells were also responsible for proportionately more of the leucine incorporation by the total heterotrophic population. While the CTC-positive cells had higher average cell-specific leucine incorporation rates than the HNA cells, their low abundances meant that they were responsible for less than 15% of the total leucine incorporation. The diversity ofBacteria observed within the HNA and LNA assemblages was examined using phylogenetic analysis based on the V3-V4-V5 variable regions of 16S rRNA genes. Most of the phylogenetic groups ofBacteria identified in this study were
present in both the HNA and LNA assemblage and had, at times, an active electron transport system (i.e., were able to reduce CTC). Finally, non-metric multidimensional scaling was presented as a new method to analyze DNA sequence data in conjunction with measured environmental parameters.